This invention relates generally to angular position measurement apparatus and, more particularly, to a system for automatically calibrating a potentiometer controlled angular position control device.
A well known angular position control mechanism employs signals obtained from potentiometers as position-sensing means. In a typical system a turntable base is driven by a cable means attached to a winch and a potentiometer is coupled to the winch. The potentiometer senses the angular position of the winch(and thus the turntable position), and produces a signal that is fed into a control device, servomechanism that controls angular position of the turntable.
A potentiometer sensing arrangement is inexpensive to produce, engineer, manufacture and maintain because of readily available, low-cost potentiometers and conventional signal processing schemes. A drawback of this arrangement, however, stems from an inherent characteristic of potentiometers. As the system is used, the potentiometers wear physically and the signal output from them deviates from an original, factory signal output. This wear leads to drift in the rotary positioning of the turntable and requires routine recalibration. An example of a potentiometer controlled system of this type is disclosed in U.S. Pat. No. 4,507,044.
Other known systems employ optical encoders to measure and control angular position. For instance, an encoder disc having slots cut into its rim at predetermined intervals may be mounted on a turntable base and used to control rotary motion thereof. Each slot represents a unique turntable rotary position. An optical encoder aimed at the edge of the disc may be used to detect these slots. In the simplest incarnation of a device of this type, a software counter can merely count the number of slots on the position disc detected by the optical encoder as a drive motor moves the turntable through its rotary movement. If the turntable starts its movement at a known "home" position, the software counter may be set to zero and the number of counts obtained by the counter will thus be indicative of the angular position of the turntable. An example of an optical encoder controlled system is disclosed in U.S. Pat. No. 2,944,157.
Optical encoder systems, however, also have limitations. To sense position at a high degree of accuracy, the slots cut into the disc must be placed very close together. This is so because the number of rotary positions the system can detect is exactly equal to the number of slots, or "indices", on the position disc. In addition, the number of possible slots on the disc is physically restricted to a finite number, limiting the positional resolution of the system.
The above described limitations of optical encoder systems can be overcome to a degree by using more than one locating disc on the turntable. For example, an assembly of concentric discs (each having a plurality of graduated slots located at the periphery) can be mounted in a stacked fashion on the turntable base. Each disc has an optical encoder aimed at the edge to detect its slots. By varying such details as the arrangement of the slots on each disc and the number of discs used, a unique and less finite arrangement of index readings from all encoders reporting to the controlling device can be created. The result is that when the turntable is commanded to travel to a particular location, it will be sent to a locating disc "address" which is based on the combination of zeros and ones read from the optical encoders aimed at each locating disc.
Although arrangements of this type have benefits because of the use of the accurate optical encoders, they are expensive to manufacture because of the use of the encoders. Also, the controlled turntable using multiple locating discs requires more engineering and material and is thus also expensive to manufacture.
The object of this invention, therefore, is to provide an improved angular position measurement and control apparatus.